1. Field of the Invention
The subject invention is directed to a seal assembly for an optical sensor employed in a high temperature environment, and more particularly, to a seal assembly for an optical sensor employed in a fuel injector of a gas turbine engine.
2. Description of Related Art
Over the past decade, there has been increased use of optical probes to monitor industrial processes taking place in harsh environments. Examples include optical pyrometers for measuring temperature and fiber optic spectrometers for measuring chemical compositions. To protect and otherwise extend the operational life of these optical probes, they are often housed in a hermetically sealed shroud. The protective shroud typically has a viewing port or window, which is commonly made of synthetic sapphire. Sapphire provides a superior electromechanical, thermal and chemical properties as compared with glass or quartz.
Optical sensors with sapphire viewing ports for monitoring combustion processes in the combustion chamber of a gas turbine engine are known in the art, as disclosed for example in U.S. Pat. No. 7,007,547 to Philipp et al., wherein a sapphire lens is soldered in place within a steel sleeve by way of a ceramic-metal mix. However, it has been determined that a ceramic-to-metal seal is very sensitive to thermal expansion, and thus not suitable for applications in which the optical sensor is located in close proximity to a combustion flame, where temperatures can reach as high as 1400° F. (760° C.).
In many optical sensors used in gas turbine engines, the sapphire window is brazed into a sealing flange or mounting fixture. For example, U.S. Pat. No. 5,929,450 to Glasheen et al. discloses an optical flame sensor for flame detection in a gas turbine engine. The sensor is disposed in a housing assembly and situated in the engine wall for detecting the presence of flame in the combustion chamber or for determining when afterburner light-off as been achieved in the nozzle section. The housing assembly includes a sapphire lens assembly disposed within and laser welded to a lens holder. The lens holder is fabricated from Kovar, a proprietary alloy which accommodates the thermal expansion of the sapphire.
While a seal between a Kovar lens holder and a sapphire window is quite effective when situated in the wall of a gas turbine engine, it would not withstand the operating temperatures that are present in close proximity to the combustion flame. For example, U.S. Pat. No. 7,334,413 to Myhre discloses an optical sensor array located within a fuel nozzle for observing combustion conditions within the combustor of a gas turbine engine. The sensor array includes a bundle of optical fibers that are located within a stainless steel guide tube. The guide tube is positioned within a viewing port formed in the leading edge surface of the fuel nozzle. A sapphire window is joined to the end of the guide tube to seal the tube and protect the fiber bundle from the combustion flame.
A conventional Kovar-to-sapphire braze seal, as disclosed for example in Glasheen et al., would be ineffective in this harsh environment. Therefore, it would be beneficial to provide a robust metal-to-sapphire seal assembly that is capable of withstanding the high temperature environment that exist in close proximity to the combustion flame within the combustion chamber of a gas turbine engine.